1. Field of the Invention
The present invention relates to a photovoltaic device including a zinc oxide substrate.
2. Description of the Related Art
Zinc oxide (ZnO) is a safe, inexpensive compound semiconductor and is known as a material that is chemically stable and also has excellent transparency. Zinc oxide is mainly used in the form of a sintered body or a powder, and is utilized for sputtering targets, varistors, rubber additives, cosmetics, and the like.
In particular, zinc oxide has also been used as a constituent material of a photovoltaic device, such as a solar cell and a photosensor, in recent years. For example, in silicon-based solar cells, zinc oxide is used as an underlying growth layer (see, for example, Patent Documents 1 to 3 and Non-Patent Document 1) or as a transparent electrode (see, for example, Patent Document 3 and Non-Patent Document 1). In CIGS (Cu—In—Ga—Se)-based solar cells, zinc oxide is used as an upper electrode (an n-type semiconductor layer) or a buffer layer (see, for example, Patent Document 4). Moreover, in ultraviolet sensors, zinc oxide is used as an underlying substrate (e.g., Patent Documents 5 and 6), as an n-type semiconductor layer (see, for example, Patent Documents 5 to 7), or as a p-type semiconductor layer (see, for example, Patent Document 5). In some cases, zinc oxide is also used as a constituent material of a photoconductive effect device (a photosensor) (see, for example, Patent Document 8).
Disclosures concerning zinc oxide in the aforementioned patent documents are, for example, as follows. Patent Documents 1 and 2 disclose that, desirably, zinc oxide used as an underlying growth layer of a silicon-based solar cell is c-axis-oriented in order for a silicon film that will be grown thereon to be preferentially (111)-oriented. Patent Documents 1 to 3 and Non-Patent Document 1 disclose that zinc oxide used as an underlying growth layer of a silicon-based solar cell is formed on a glass or like support substrate generally by using a sputtering method. Patent Document 8 discloses that zinc oxide by a commonly used sputtering method grows in a microcrystalline state to have a grain diameter of about 50 to 100 nm. Patent Document 2 and Non-Patent Document 1 disclose that, desirably, the silicon film of a silicon-based solar cell is composed of large silicon grains for enhanced photoelectric conversion efficiency. Patent Document 4 discloses that in a CIGS-based solar cell, a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode are formed in this order on an underlying substrate. Patent Document 4 discloses that ZnO used as the upper electrode in the CIGS-based solar cell functions as a transparent electrode as well as an n-type semiconductor layer. Patent Documents 5 and 6 disclose that zinc oxide used as an underlying substrate of an ultraviolet sensor is generally single-crystal zinc oxide. Patent Document 7 discloses that in some cases, a glass plate, a metal plate, or the like is used as an underlying substrate of an ultraviolet sensor, and a semiconductor functional layer is formed thereon.
Patent Document 10 discloses a method for producing an oriented polycrystalline zinc oxide sintered body in which the a-axis or the c-axis is oriented in a predetermined direction. According to this document, a rotating magnetic field is applied to a slurry, in which hexagonal zinc oxide crystal particles are dispersed, to cause the crystal particles to be oriented, then the slurry is solidified by drying to prepare a green body, and this green body is sintered in an oxygen-containing atmosphere to produce an oriented polycrystalline zinc oxide sintered body.